An electron may have not only energy and momentum, but also spin. The direction of spin relative to a specified direction represents an important piece of information that can be determined by means of the interaction of the electron with other particles, for example with atoms in a surface layer of a solid. For carrying out such an investigation, apparatus for measuring the degree of spin polarization of electron beams is necessary. Such equipment is already known that operates on the basis of the dependence upon the electron spin of the scattering of electrons by free atoms, the so-called Mott scattering. In such equipment, determination of the degree of polarization of the incident electron beams is made from the comparison of intensity measurements at scattering angles that are equal in amount but of opposite sign. There is a disadvantage in this case, however, that appreciable intensity differences resulting from the electron spin direction occur only at low overall intensity. The sensitivity of the known equipment is therefore very limited. This is all the more significant a disadvantage, because strong polarization effects are limited to only small scattering angle ranges. That has the consequence that only a very small part (.ltorsim.10.sup.-4) of the aggregate quantity of scattered electrons can be used for measurement.
The known measuring equipments for this purpose are based on the application of two different methods. In the first method, an atom beam, preferably of Hg, is used, against which the electrons are scattered with a few keV of energy. Since it is difficult, however, to produce atom rays in high density, difficulties occur in this kind of method regarding the intensity of the radiation. In the second method, thin foils are used for scattering the electrons. In this case, however, with the advantage of high atom density, there is the unavoidable disadvantage of multiple scattering of the electrons and their absorption in the foil. In order to cope with these difficulties, the measurement has therefore been carried out with electrons accelerated to high energy (100 to 150 keV). In that case, there is still the disadvantage that the effective cross-section for the scattering is very small and therefore only a small scattering intensity and measurement sensitivity is obtained. The application of high electric voltages, in addition, makes the known equipment very large and cumbersome on account of the necessary safety precautions.
It has therefore already been recommended that the degree of polarization of an electron beam should be measured by means of the spin-dependent intensity of an electron beam specularly reflected by the surface lattice of a monocrystal. The differential effective cross-section for the refraction of slow electrons dependent upon the orientation of the electron beam is in this case higher by several orders of magnitude than in the case of the Mott scattering. The sensitivity is also correspondingly higher. Since in the case of the measurement of the degree of polarization the scattering must always be carried out at two complementary and as nearly as equal as possible angles in the incidence plane of the beam, it has been favored to measure the two scatter beams by tipping the crystal alternately by equal angles, first to one and then to the other side, and measuring the scatter intensity after each change (see in this regard R. Feder, Surf. Sci. 51, 297, 1975). Much time and expense, and therefore disadvantage, is involved, however, because the mechanical movement of the crystal must be carried out with high precision and reproducibility, very often, and with sufficient rapidity and, moreover, in ultrahigh vacuum (10.sup.-11 mbar). The time consumption required for carrying out such a double measurement is more than twice as high as required for a single measurement.